London ground to a halt Wednesday as workers on the city’s underground train system launched a series of strikes.

“Tube” drivers have walked off the job two days this week and plan to do so again next week, in protest at job cuts and changes to the system. CBS News reports about 3 million people use London Underground, the city’s main public transit rail service, every day. Commuters were forced to take buses, walk, and bicycle to work.

Prime Minister David Cameron has condemned the strikes, calling them “shameful” on Twitter, and later saying in parliament, “There is absolutely no justification for a strike. We need a modernized tube line working for the millions of Londoners who use it every day.”

Two of the rail workers’ unions have planned a series of strikes in protest of job losses and the closing of manned ticket offices, according to CBS News. The strike could reportedly cost the city’s economy $326 million.